One prior nonvolatile semiconductor memory is the flash electrically erasable programmable read-only memory ("flash"). Flash memories are programmed electrically and, once programmed, retain data until erased. After erasure, flash memories may be programmed with new code or data.
Flash memories differ from conventional electrically erasable programmable read-only memories ("EEPROM") with respect to erasure. Conventional EEPROMs typically use a select transistor for individual byte erasure control. Flash memories, on the other hand, typically achieve much higher densities using single transistor cells. Some prior flash memories are erased by applying a high voltage to the source of every memory cell in the memory array simultaneously. This results in the full array erasure.
Flash memory conventions define a logical one as few, if any, electrons stored on the floating gate of a memory cell. Convention also defines a logical zero as many electrons stored on the floating gate of the memory cell. Erasure of the flash memory causes a logical one to be stored in each cell. Flash memory cells cannot be overwritten individually from a logical zero to a logical one without prior erasure. However, a flash memory cell can be overwritten individually from a logical one to a logical zero, because this entails simply adding electrons to a floating gate that contains the intrinsic number of electrons associated with the erased state.
Erasure, programming and verification of flash memories requires careful control of the voltages required to perform those operations. One prior flash memory is the 28F008SA complimentary metal oxide semiconductor (CMOS) flash memory sold by Intel Corporation of Santa Clara, Calif., which is an 8 megabit flash memory. The 28F008SA flash memory includes a command register to manage erasure and programming. The command register contents serve as input to an internal state machine that controls erasure and programming circuitry.
The 28F008SA implements a test mode, which permits verification of its internal operation. The test mode is accessed by pulling one of the device's control pins to an abnormally high voltage, as is the common practice. For example, in the 28F008SA test mode is accessed by pulling a control pin to 12 volts. While the control pin is held high, the command register recognizes test mode commands that would not ordinarily recognized or executed. A disadvantage of this method of accessing test mode operation is that the abnormally high voltage may not be available in systems, so that verifying device operation requires removing the device from the system. This is increasingly a problem because many flash memory devices are sold in systems, such as memory cards or solid state disks.